The present invention relates to an electromagnet drive device operating, at high speed, an electromagnet device comprising a pair of electromagnets and a movable member moved by the electromagnetic force thereof. More precisely, this invention relates to an electromagnet drive device which is ideal for use in, for example, an electromagnetic valve for carrying out accurately fuel injection in a diesel engine or electromagnetic shut off valve.
The present invention includes a high speed control means whereby operation can be described beginning at a first state in which the first of a pair of coils is energized, and a movable element is magnetically held by that first coil. When that magnetic holding is desired to be released, and the movable element is desired to be held by the second coil instead of the first coil, the following sequence is used. First, with the first coil still in a holding state, the second coil is energized. When the attachment is to be switched over from the first coil to the second coil, the first coil is deenergized. When the magnetic holding of the first coil is to be removed, and instead of the movable member is to be held by the second coil, thus, with the state of holding by the first coil preserved, the energizing current flowing through the first coil is increased and the second coil is energized. Then, when the holding is to be switched over from the first coil to said second coil, the first coil is deenergized. Therefore, the movable element can be operated at high speed using a relatively low supply voltage.
FIG. 6 is an electric circuit diagram showing the construction of a typical conventional electromagnet drive device. A pair of electromagnets have their respective structural elements which are the coils SR and SL connected in parallel to a direct current supply E, and a relay switch RL is provided to switch the voltage supply between them. In parallel with the coils SR and SL are connected diodes CR and CL, biased in the opposite polarity to the energizing current applied. FIG. 7 is a timing chart of the electromagnet drive device of FIG. 6. In FIG. 7, (1) is the switchover command signal applied to the relay switch RL, (2) is the voltage waveform applied to the coil SR, (3) is the waveform of the current flowing through coil SR, (4) is the voltage waveform applied to the coil SL, and (5) is the waveform of the current flowing through coil SL. When the switchover command signal is low level, the contacts 1a of the relay switch RL are open, the contacts 1b are closed, and the coil SL is energized. When the switchover command signal changes from low level to high level, the relay switch RL switches over, the coil SR is energized and the coil SL is deenergized. At this point, the current IR flowing through the coil SR is, if the resistance component of the coil SR is taken as R1, the inductance component as L1, and the time as t, given by expression 1 as follows. ##EQU1##
In other words, even when a voltage is applied to the coil SR, for an interval from the instant when the voltage is applied there is a transient state, and as shown in FIG. 7 (3), after the time interval T1 has elapsed, the current IR has reached its normal state. The current IL caused by the back e.m.f. produced when the coil SL is deenergized is conducted by the diode CL to flow as shown by the arrow A1, and is dissipated by the resistance component R2 of the coil SL. At this point, if the inductance component of the coil SL is L2, the current IL is given by expression 2 as follows. ##EQU2##
The voltage applied to the coil SL becomes zero, and the current IL produced by the back e.m.f. gradually decreases, as shown in FIG. 7 (5), over interval T2.
When an electromagnet drive device of this type drives a double solenoid type electromagnetic valve as described below and shown in FIG. 2, the movable element which is the switchover valve spool and the plungers of the pair of electromagnets are subject to a force. This force is the difference between the electromagnetic attraction force produced by the current IR flowing through the coil SR, and, working against this, the electromagnetic attraction force produced by the current IL flowing through the coil SL when the switchover signal switches from the low level to the high level. When the force produced by IR exceeds the latter the movable element moves from the side of the right hand coil SR to the side of the left hand coil SL in time interval T3 as shown in FIG. 7 (6). Also, when the switchover signal switches from the high level to the low level, the coil SL is energized and the coil SR is deenergized, whereupon the movable element is moved to the right by the action of the force which is the difference between the electromagnet attraction force of the coil SL and the electromagnet attraction force of the coil SR.
For the movable element to be moved at high speed by electromagnetic force as described above, the current flowing through the energized coil must be increased rapidly, and also the current flowing through the deenergized coil must be decreased rapidly. Since, however, there is an inductance component in the coils, the rate of change of the respective currents is limited, and it is not possible to make the time interval T to move the movable element much shorter. From expression (1) above, if the supply voltage is increased, the energizing current apparently rises rapidly. However, in practice since the normal value of the energizing current also increases, the coil temperature rises, and the resistance component increases, so that no great benefit can be obtained. If, moreover, in an attempt to increase the speed of movement of the movable element an excessive voltage is applied to the coil, the coil will burn out, so that there are limits to increasing the supply voltage.
FIG. 8 shows an example of typical electromagnet attraction force characteristics. The relation between the electromagnet attraction force F and the electromagnet gap .delta. which causes the effective magnetic flux contributing to the attraction force is shown with the energizing current I as a parameter. In the drawing, .delta. max is the maximum value of the gap .delta., and .delta. min the minimum value of the gap .delta.; the energizing currents I are such that Ia is less than Ib which is less than Ic which in turn is less than Id. When the energizing current I is Ib, when the gap .delta. is the maximum value .delta. max, the attraction force is taken as Fa, and when the gap .delta. is the minimum value .delta. min the attraction force is taken as Fb. A pair of electromagnets having such characteristics is used as follows, in for example the construction of a double solenoid electromagnetic valve of FIG. 2 described later. That is, when the first electromagnet is attracting its plunger with the gap .delta. min, it will be producing the attraction force Fb. The second electromagnet coil is energized with the gap .delta. max or that it will be producing the attraction force Fa. At this point, if the difference (Fb-Fa) of the attraction forces is greater than the external forces acting on the switchover valve spool, for example a spring force or fluid force (that is forces acting on the spool as a result of fluid flow), then the spool will remain moved to the side of the second electromagnet, and that state will be preserved. From this state, if the current energizing the coil of the first electromagnet is suddenly removed, the attraction force Fb will disappear, and only the attraction force Fa will act on the spool. Thus the spool will move at high speed to the side of the first electromagnet.
According to the present invention, on the occasion of changing from the state in which the first coil is energized and a movable element is magnetically held by that first coil, to the state where the movable element is held by the second coil, certain operations are followed. Specially, while still in the state that the holding of the first coil is maintained, the second coil is energized, and the first coil is deenergized. Similarly, when switching to holding by the second coil, the energizing current of the second coil is increased and the first coil is deenergized. When switching over the holding, while still in the state of maintaining the holding of the first coil, the energizing current of the first coil is increased and after energizing the second coil, the first coil is deenergized. The present invention also encompasses a circuit being provided for each coil, comprising a diode connected with opposite polarity to the polarity of the energizing current, and a nonlinear element which conducts when the coil back e.m.f. exceeds a certain predetermined value.